1. Field of the Invention
This invention relates to an improvement of a noncontact type tonometer which is designed to improve the reliability of intraocular pressure measurement while taking into consideration the fluctuation of intraocular pressure due to pulse fluctuation. More particularly, it relates to a noncontact type tonometer including means for discharging fluid toward the cornea of an eye to be tested (hereinafter simply referred to as the "eye") upon receipt of a projection signal, and means for detecting the pulse wave of a patient and outputting alternating voltage corresponding to such detected pulse wave.
2. Description of the Related Art
As a conventional tonometer, there is a tonometer of a noncontact type, such as an air puff type, wherein a fluid pulse is discharged toward the eye of a patient and intraocular pressure of the eye is measured from the relation between the transfiguration of the cornea of the eye and the pressure of the fluid discharged. In the tonometer of this type, an air pulse is instantaneously discharged toward the eye in a short period of time such as, several tens of milliseconds and the cornea is transfigured (for example, applanation) in such a very short period of time as several ms to measure the intraocular pressure.
In general, intraocular pressure fluctuates in accordance with the fluctuation of the pulse. The maximum fluctuation of the intraocular pressure is several mmHg. The intraocular pressure of a normal human eye is usually 10 mmHg.about.20 mmHg. Also, the human pulse is usually 60 times/min..about.120 times/min., and the cycle of the fluctuation of the intraocular pressure is at least approximately 50 ms. Therefore, if the measurement of the intraocular pressure is carried out without taking into consideration the fluctuation of the intraocular pressure due to the pulse, the value of such measured intraocular pressure could become correspondingly higher if the measurement is carried out at the peak of the pulse wave. Similarly the value of such measured intraocular pressure could become correspondingly lower if the measurement is carried out at the valley of the pulse pressure wave (sphygmogram). Therefore, if the fluctuation of the intraocular pressure due to the pulse is not taken into consideration, the reliability of the measurement would become lower.
Therefore, there has been proposed a tonometer for measuring intraocular pressure while adjusting for the pulse (U.S. Pat. No. 3,572,100 issued Mar. 23, 1971). The tonometer disclosed in this Patent Publication is designed such that the measurement of the intraocular pressure is always carried out at the same phase position of the pulse wave. According to this tonometer, measuring errors caused by the fluctuation of the pulse can be removed.
In this tonometer, however, since one cycle of the pulse is divided into five equal parts and the measurement of the intraocular pressure is carried out in synchronism with a single selected phase position out of such divided five equal parts, the measurement cannot be carried out in synchronism with a desired phase position other than such divided five equal phase positions.
Generally speaking, a noncontact type tonometer requires strict accuracy in alignment of the tonometer with respect to the eye. In order to carry out the measurement with high accuracy, for example, the vertical or horizontal position of a fluid discharging nozzle (hereinafter simply referred to as the "nozzle") with respect to the eye, i.e., the working distance from the eye to the tip of the nozzle, requires strict accuracy in alignment. The tolerance in alignment is required to be less than one mm from a correct alignment position. With the above-mentioned tonometer, however, since the measurement of the intraocular pressure is carried out in synchronism with the selected phase of the pulse irrespective of correctness of the alignment, the measuring accuracy is lowered.